The invention relates to an apparatus and a method for detection and demodulation of an intensity-modulated radiation field.
The behavior of many systems in technology is investigated, inter alia, with the aid of modulated signals. The system is in this case excited with a modulated signal, for example a sinusoidal signal, and the response of the system is measured. The modulation of the system response obtained, its phase shift with respect to the exciting signal, and the background signal level (offset) are defined as characteristic variables.
In the case of known semiconductor image sensors, two-dimensional distributions of the light intensity are converted into two-dimensional photoelectric current density distributions. The light-generated signal charges are integrated with respect to time in so-called pixels. For example, a CCD image sensor is disclosed in DE 39 09 394 C2, in which the charge pattern generated is shifted laterally during the exposure. This is intended to avoid the occurrence of movement blurring when recording objects which are moving relative to the image sensor.
A method is known for use in non-scanning, imaging laser-radar 3D cameras, in which modulated light is imaged on a conventional image sensor (Laser-Radar Imaging Without Scanners, Photonics Spectra, (28 Apr. 1994). The demodulation is carried out using an intensifier element, which receives images and is variable with respect to time, between the imaging objective and the semiconductor image sensor. The intensifier element is designed as a microchannel plate (MCP), it being necessary to operate at high voltages of 100 to 1000 volts. The incoming light is absorbed in the intensifier element, being modulated with respect to time, and then passes to the image sensor, the latter having only the function of an integrator. Three or more images can be recorded in this case, it being necessary to accept a considerable loss of light because of absorption in the intensifier element. In addition, the images must be read out of the image sensor completely between the recordings.
In addition, a CCD image sensor is known for the demodulation of time variable polarized light (H. Povel, H. Aebersold, J. O. Stenflo, "Charge-coupled device image sensor as a demodulator in a 2-D polarimeter with a piezoelastic modulator", Applied Optics, Vol. 29, pp. 1186-1190, 1990). A modulator is arranged between the objective and the CCD image sensor for this purpose, which modulator changes the polarization of the light between two states in rapid sequence. The two images produced of the two polarization states are accumulated and stored in the image sensor. A known image sensor is for this purpose provided with a strip mask, which covers every other image sensor line in a light-proof manner. In this way, the image of the respective polarization state can be accumulated in the correct cycle by shifting the image charge pattern up and down vertically.